Low-energy El spectra

Nonetheless, during the first decades of analytical mass spectrometry low energy El spectra have been the only way to minimize fragmentation, and thereby to increase the relative intensity of a weak molecular ion peak. Nowadays, El mass spectra are preferably complemented with spectra obtained from so-called soft ionization methods (Chaps. 7-11). 

Figure 3.70 Room temperature optical absorption spectra of a 45tX> A-thick film of neutral polypyrrole doped with 12 at 0.03 torr, (a) before exposure to l2. conductivity < IO-6fl-1 cm - (b) after 2 minutes Ij exposure, conductivity 4.80" em-1 (c) after 7 minutes exposure, conductivity 6.7fi-,cm-1 (d) after 22 minutes la exposure, conductivity 32ft- cm. The three structures seen on the low-energy side of (a)- (c) arc possibly artifacts due to interference effects in the films. From Pfluger el ill, (1983).

Intermolecular isotope effects have been studied in the El mass spectra (at low energies of 11—35eV) of variously deuterated methanols [197]. Studies of metastable ion decompositions in deuterated methanols revealed small intermolecular isotope effects for H atom loss [76, 536]. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

Comparisons between MS/MS of the (M - HJ- and /M + H]+ Ions of Oligpdeoxynudeotides. Under low-energy CID conditions in the ion-trap, similar classes of ions, such as the base loss nonsequence and the wn sequence ions, are observed in the MS/MS spectra of both the [M - H] and [M + H]+ ions [44]. The key physical organic concepts discussed above can readily be used to provide mechanistic rationals for these fragmentation reactions. For the M + H]+ ions the most likely site of protonation is at the nucleobase sites, which have the higher proton affinity. Thus protonation at these sites acts as a trigger for neutral base loss via El or intramolecular E2 mechanisms [44]. The order of nucleobase loss for the [M + H]+ ions is C > G > A > T. In contrast, for the... 

Conventional mass spectra may be obtained in El at low energy (14 eV) and can be valuable for locating abundant molecular ions from a mixture. Levsen and Schulten [253] thus studied the mixture furnished by the pyrolysis of DNA (after heating a herring DNA sample to 600°C). The El spectrum obtained is very complex (Fig. 103). 

In Figure 6, PL spectra and images of blended films with different relative donor/acceptor concentrations, spin-coated on quartz substrates, are reported. It is seen that enhancing the concentration of STO a broad red-shifted emission due to exciplex states appears, in addition to the blue emission due to TPD, which is responsible for the white emission within a concentration range 17-53% of STO in TPD. The normalized EL spectra were similar to the PL spectra for the concentration used (20%), showing that the shape of the low-energy exciplex spectrum is almost independent of the applied voltage. The CIE coordinates of the EL spectra indicated a balanced white emission (0.39, 0.40) (Mazzeo et al., 2003 b). 

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